1. Field of the Invention
This invention pertains generally to transfer mechanisms and, more particularly, to transfer switches for selectively feeding power from one of two input lines to a load. The invention also pertains to power systems which include a transfer mechanism, such as a transfer switch.
2. Background Information
Alternate power sources are provided for any number of applications, which cannot withstand a lengthy interruption in electric power. Typically, power is provided from a primary source with back-up power provided by a secondary source. Often, the primary source is a utility power source and the secondary source is an auxiliary power source, such as an engine driven generator or a second utility source. The transfers between the two power sources can be made automatically or manually.
Transfer switches are well known in the art. See, for example, U.S. Pat. Nos. 5,397,868; 5,210,685; 4,894,796; and 4,747,061. Transfer switches operate, for example, to transfer a power consuming load from a circuit with a normal power supply to a circuit with an auxiliary power supply. Applications for transfer switches include stand-by applications, among others, in which the auxiliary power supply stands-by if the normal power supply should fail. Facilities having a critical requirement for continuous electric power, such as hospitals, certain plant processes, computer installations, and the like, have a standby power source, often a diesel generator. A transfer switch controls electrical connection of the utility lines and the diesel generator to the facility load buses. In many installations, the transfer switch automatically starts the standby generator and connects it to the load bus upon loss of utility power, and reconnects the utility power to the load bus if utility power is reestablished.
In the case of a generator driven auxiliary power source, power must be stabilized before the transfer can be made to the secondary source. In any event, the two power sources cannot be connected to the load simultaneously unless they suitably match their respective voltages, frequencies and phases. Some transfer switches affect an open transition between the power sources, that is, one is disconnected from the load bus before the other one is connected. Other transfer switches provide a closed transition wherein the one source is connected to the load bus before the other source is disconnected, in order that both power sources are connected in parallel during the transition.
Transfer switches commonly used to connect alternate power sources to a load, including networks, utilize a pair of switches each connecting one of the sources to the load. In order to prevent connecting unsynchronized sources together, the operation of the two switches is coordinated, typically by a mechanical interlock, in order that only one switch at a time can be turned on. In many instances, it is desirable to operate the transfer switch remotely. Typically, electric motors have been used to operate the interlocks on transfer switches. See, for example, U.S. Pat. Nos. 5,081,367; 4,760,278; and 4,398,097.
A transfer switch typically comprises a pair of circuit interrupters combined with a drive input and a linkage system. The preferred types of circuit interrupters have been molded-case switches and molded-case circuit breakers because these types are commercially available in a wide array of sizes and are relatively economical compared to other options. The preferred type of drive input depends on the application for the transfer switch. Usually motors are preferred, but at other times there is a clear preference for manually-operated mechanisms.
Transfer switches as applied, for example, to light commercial and residential applications are becoming popular since a number of such commercial and residential facilities find it necessary to install backup power generation systems. Furthermore, recent revisions to the National Electrical Code (NEC) require transfer equipment on all permanently installed optional standby systems. Many such transfer switches include fixed timers, voltage sensing and engine start contacts.
Underwriters Laboratories (UL) standard UL 1008 covers transfer switch equipment including automatic, non-automatic (manual) and by-pass/isolation transfer switches intended for use in ordinary locations to provide for lighting and power. Relatively low cost transfer switches that meet this standard design are suitable for many light commercial and residential applications.
It is known to provide transfer switches for dual utility power sources.
It is also known to provide a transfer switch for a utility power source and a generator power source. For example, U.S. Pat. No. 6,181,028 discloses a transfer mechanism for a utility power source and a generator power source. A monitoring circuit within a transfer mechanism cabinet is operatively connected to the utility power source and the generator power source. As is conventional, the monitoring circuit monitors the power supplied by the utility power source. In response to a power outage from the utility power source, the monitoring circuit starts the internal combustion engine of the generator power source. The starting of the internal combustion motor causes the electrical generator of the generator power source to generate electrical power.
It is also known to provide a voltage sensing generator (i.e., a generator with a voltage monitoring circuit).
U.S. Pat. No. 6,172,432 discloses an automatic transfer switch in which excess loads are automatically shed and restored when an intermittent load combination falls back to acceptable limits as configured into a microcontroller. For example, microwave ovens, water pumps and air conditioning compressors create large but intermittent and transient loads. When such loads turn off, the microcontroller automatically restores previously cut out lower priority loads. Load shedding avoids the necessity of oversizing the generator and permits a larger combination of loads to be supported by the generator than the total load rating of the generator.
U.S. Pat. No. 6,191,500 discloses that an uninterruptible power supply (UPS) system includes a plurality of UPS modules electrically coupled to at least two utility sources. Generator paralleling switchgear includes a load priority and load shedding control system that adds or reduces load on a generator bus. Each load in the UPS system has a pre-assigned wattage rating, which the control system employs to determine how many loads can be added as generators come onto the generator bus. As generators are randomly connected to the generator bus, the control system signals for connection of the loads in ascending sequential priority, with the highest priority load being connected first. All load-add steps are preceded by a time delay, adjustable from 0 to 3 seconds.
There is room for improvement in powering loads under emergency or generator back-up power conditions.
There is a need for a low cost transfer switch for use with a utility source and a voltage sensing generator, or with two generators, one of which is voltage sensing.
There is, therefore, room for improvement in transfer switches. There is also room for improvement in power systems including transfer switches.